Upgrade a Magento platform to 1.6

We have a Magento 1.4.2 installation and we are looking to upgrade it at 1.6 (I hope to have a 1.6.1 when this project will start. - A custom extension for manage attributes (on the admin and the frontend side - example: [login to view URL]) - Unirgy_Giftcert - EM_DeleteOrder - AW_Blog - Fooman_Speedster - Ebizmarts_MailChimp - Image option Theme - Hellowear ([login to view URL]) We have made some various stupid customizations on the theme side, but I cannot remember all of them. We are asking you to copy the actual website on a your environment in which you'll made the upgrade. We and you will testing this upgrade and fix the potential errors. When we'll be sure that everything will be stable, we'll upgrade also the production environment (obviously we are live and so the production environment will continue to work and use their data). We are asking you, also, to support us for some days after the go-live directly on the production environment. We also glad if you will help us to optimize a little bit our website, being pro-active!